The invention relates to a method of coloring cut gemstones by introducing metals or metal oxides into a surface layer by means of heat treatment.
Surface coloring of gemstones with transition metals is a procedure which has been repeatedly described since early times. In more recent times endeavours in particular have been made to reduce the temperature involved in the heat treatment in order to be kind to the surface of the stones. That was achieved for example by a procedure whereby the cut stones were not vapor deposited with cobalt oxide alone but with cobalt in combination with aluminum oxide (see DE-A-22 29 909). Balitskii (see U.S. Pat. No. 3,950,596) stated that not all oxides have to be applied to a polished gemstone but it is possible to use the stone itself as one of the reactants. Surface coloring can therefore be effected both by diffusion and also by reaction.
A series of important minerals which are used for gemstones are suitable for the coloring effect. Besides corundum these are inter alia the various spinels, the island silicates olivine, zircon, topaz and garnet, the aluminosilicates such as andalusite, disthene or mullite, and the ring silicates beryl and cordierite which can all react with cobalt.
The difficulty with all these diffusion and reaction methods lies in the way in which the reactants are brought together and the way in which the reaction is controlled in respect of temperature and time in such a way that the polished surface of the stones is not damaged.
A first variant of the method involves bringing the stones into close contact with metal or metal oxide powder and heating them. That suffers from the disadvantage that the partially poisonous metals or metal oxides have to be handled. The stones first have to be carefully embedded. The procedure involves using suitable dishes for the metal oxides, and they take up a very great deal of furnace space. The stones have to be cleaned after the treatment and the coloring substances used have to be processed in an expensive manner in view of the environmental protection aspects in order to be able to use them further. The risk of stones being damaged is also great due to the cleaning operation involved.
The alternative method of applying metals or metal oxides to gemstones by vapor deposition or sputtering is substantially cleaner. The disadvantage here is that the procedure is a two-stage process. After the coating operation the coated stones have to be extremely cleanly stored as even just very slight impurities prior to the thermal treatment can result in burning-in phenomena when the treatment is carried out.
Both methods have a common disadvantage: they frequently involve direct solid-body reactions which do not take place uniformly on the entire surface to be colored. That also results in local damage to the polish.